I actually got lucky by just throwing knives randomly at a group and it happened to pop. However, steam user WolfauraRose posted a much better and more efficient way of doing this. She/He posted to "
Do a custom battle with 100% archers 200 army both sides and use the sarranid girl with jarids. Approach the enemy army and from being far away as in just seeing the enemy, throw the jarrid into the 200 mass army and you'll get a 5 difficulty shot. Distance is the key.". This way is certainly more eloquent than mine. And may net the achievement quicker.
NOTE: The comment was posted here
http://steamcommunity.com/sharedfiles/filedetails/?id=321298...